Pure cultures and growth conditions

Solid media are used to isolate and store bacteria; liquid broth is used for fast, large-scale growth. To study a microbe, we first need a pure culture: a culture of one strain only. The idea in every method is to dilute the sample until single cells land apart, each growing into its own colony.

• Streak plate: a flamed, cooled loop is used to streak the sample across an agar plate in several sections, so it thins out. Towards the end, isolated colonies grow.
• Pour plate: the sample is diluted step by step (serial dilution: 1 mL into 9 mL of saline, again and again). 1 mL of a dilution is put in a dish, melted agar is poured over it, and the dish is swirled to mix.
• Spread plate: 0.1 mL of a dilution is spread over the agar surface with a sterile L-shaped glass spreader.

Then give the microbe the conditions it likes.

Temperature. Growth rises with temperature up to an optimum, then falls quickly. Mesophiles grow best at about 20-45 °C (most microbes, including those living in our bodies). Thermophiles grow at about 40-80 °C (optimum 50-65 °C). Extreme thermophiles can grow above 100 °C.

pH. Most microbes like a pH near 7. Many bacteria prefer slightly alkaline conditions; fungi prefer slightly acidic ones, and tolerate a wider range.

Oxygen.

• Aerobes need oxygen; in flasks of broth they are shaken or stirred to dissolve enough of it.
• Obligate anaerobes cannot tolerate oxygen, and are grown in anaerobic chambers.
• Facultative anaerobes grow with or without oxygen.
• Aerotolerant anaerobes do not use oxygen, but are not harmed by it.

Autotrophs get their CO₂ as bicarbonate in the medium or from CO₂-enriched air; phototrophs also need light of the right wavelength.